Most telephone sets draw DC current from the subscriber loop that connects them to the central office (CO). For satisfactory operation, the telephone set requires a current in the range of 18 mA to 50 mA. The DC loop current is provided by the central office battery, whose terminals are typically coupled to the subscriber loop via two battery feed resistors. The DC resistance measured between the TIP and RING of the subscriber loop (including the telephone set) typically is in the range of 100 to 1900 ohms and depends upon the length of the subscriber loop. To provide sufficient loop current for telephone operation with the longest subscriber loop, the value of the battery feed resistors is typically limited to 200 ohm each. In this case, the loop current is I.sub.loop =52V/(400+1900)=22.6 mA, which is above the minimum current required for proper operation. In the case of a short subscriber loop, one having a resistance of 100 ohm, the loop current is I.sub.loop =52V/(400+100)=104 mA, which exceeds the desired range of loop current for proper telephone set operation.
It is desirable to provide a battery feed arrangement capable of limiting the loop current in the case of a short subscriber loop. A circuit to limit the loop current would have to meet the following requirements: provide variable DC resistance to limit the loop current; provide low AC impedance to meet the voice frequency requirements of the subscriber line interface; and withstand, without further protection, AC signals appearing at the TIP and RING and resulting from lightning strikes on the loop, AC power conductor induction, and AC power conductor short-circuit.
There have been attempts to introduce current limit circuits with the battery-feed. For example, John M. Hurt in U.S. Pat. No 4,560,834 issued Dec. 24, 1985, teaches an active current limiter in only the ring side of a telephone circuit. While the arrangement taught by Hurt successfully limits loop current to the desired range, the presence of certain components within the arrangement may prevent it from meeting all of the requirements listed above.